The present invention broadly relates to a method for treating a copper or copper alloy substrate to improve resistance to both oxidation and chemicals. More particularly, the substrate is immersed in an aqueous solution containing chromium (VI) ions and phosphate ions.
Copper and copper based alloys are widely used in electrical and electronic applications. Among the more widespread electronic uses are the manufacture of leadframes from strip and the manufacture of conductive circuit traces from foil. The foil is either wrought, produced by mechanically reducing the thickness of a strip such as by rolling, or electrodeposited, produced by electrolytically depositing copper ions on a rotating cathode drum and then removing the deposition from the drum. The foil is bonded to a dielectric support layer which is either rigid such as FR-4 (a flame retardant epoxy), or flexible such as a polyimide. After lamination, circuit patterns are formed in the copper foil by selective etching.
Copper is a material of choice for electronic applications due to high electrical conductivity. One drawback with copper and its alloys is reactivity. The metal reacts with oxygen and tarnishes. The metal is also reactive with some of the chemical solutions encountered during the manufacture of electronic components such as hydrochloric acid and sodium hydroxide. Many anti-tarnish coatings to prevent oxidation of a copper foil prior to lamination are known. These coatings do not also impart improved chemical resistance, particularly after lamination.
Some typical anti-tarnish coatings, all of which are assigned to the same assignee as the present application and all of which are incorporated by reference in their entirety herein, include (1) mixtures of chromic acid and phosphoric acid; (2) sodium dichromate and phosphoric acid; and (3) a co-deposited layer of chromium and zinc.
U.S. Pat. No. 3,837,929 to Caule discloses an aqueous solution containing 3.5 grams per liter (g/l) to saturation of sodium or potassium dichromate mixed with 8-85% phosphoric acid (83-1436 g/l). A foil is immersed in the solution for at least two seconds and then rinsed. Preferably, as disclosed in U.S. Pat. No. 3,764,400, also to Caule, rinsing is in an alkaline solution (PH 8.5-11) at a temperature above 90.degree. C. As further disclosed in U.S. Pat. No. 3,941,627, also to Caule, the treated foil is then laminated to a substrate with an adhesive.
The Caule coating provides the foil with good anti-tarnish resistance. However, the coating is applied prior to lamination and imaging of the foil to form lead traces. A necessary characteristic of the coating is limited acid resistance as disclosed in the '627 patent. The anti-tarnish coating is dissolved in an acid prior to imaging of the foil.
Another coating is disclosed in U.S. Pat. No. 4,647,315 to Parthasarathi which discloses a solution containing 0.02 to 1 g/l chromic acid and 0.02 to 1 g/l phosphoric acid. The solution is used at a temperature of from 60.degree. C. to 90.degree. C. and the foil immersed for 1-120 seconds.
The above coatings are applied non-electrolytically. An electrolytic coating comprising a co-deposited layer of chromium and zinc is disclosed in U.S. Pat. No. 5,022,968 to Lin et al. The substrate is immersed in an aqueous basic solution containing hydroxide ions, from 0.07 g/l to 7 g/l zinc ions and from about 0.1 g/l to 100 g/l of a water soluble hexavalent chromium salt. The concentration of either the zinc ions or the chromium (VI) ions or both, is less than 1.0 g/l. An electric current of from about 1 milliamp per square centimeter to about 1 amp per square centimeter is impressed across the electrolytic cell with the substrate forming the cathode. The electrolytically deposited coating provides an anti-tarnish coating readily removed in both dilute hydrochloric and sulphuric acids.
The above treatments are well suited for protecting a copper or copper alloy substrate prior to photoimaging. It is also desirable to provide an anti-tarnish coating to protect imaged circuit traces laminated to a printed circuit board from oxidation, exposure to chemicals, temperature cycling, and contaminants such as fingerprints, soils and residual chemicals. Unfinished printed circuit boards are frequently stored before being further processed and improved performance is achieved by minimizing tarnish during storage. The printed circuit boards may be multi-layer with a plurality of planar circuit traces separated by dielectric layers. The anti-tarnish coating of the invention enhances adhesion between circuit traces and the dielectric layer.